U.S. patent application number 16/973301 was filed with the patent office on 2021-08-12 for conductive bridge memory device, manufacturing method thereof, and switching element.
This patent application is currently assigned to NATIONAL UNIVERSITY CORPORATION TOTTORI UNIVERSITY. The applicant listed for this patent is NAGASE & CO., LTD., NATIONAL UNIVERSITY CORPORATION TOTTORI UNIVERSITY. Invention is credited to Toshiyuki ITOH, Kentaro KINOSHITA, Shigeki MORII, Toshiki NOKAMI.
Application Number | 20210249595 16/973301 |
Document ID | / |
Family ID | 1000005610039 |
Filed Date | 2021-08-12 |
United States Patent
Application |
20210249595 |
Kind Code |
A1 |
ITOH; Toshiyuki ; et
al. |
August 12, 2021 |
CONDUCTIVE BRIDGE MEMORY DEVICE, MANUFACTURING METHOD THEREOF, AND
SWITCHING ELEMENT
Abstract
A high-performance CB-RAM having a low operating voltage and a
high switching endurance even when alumina is used for the
insulator layer; wherein, an electrolyte layer impregnated in a
pore of an insulator layer is configured to include a mixed ionic
liquid in which is mixed a solvate ionic liquid, and a low-viscous
ionic liquid being an ionic liquid having a viscosity coefficient
smaller than that of the solvate ionic liquid.
Inventors: |
ITOH; Toshiyuki;
(Tottori-shi, JP) ; NOKAMI; Toshiki; (Tottori-shi,
JP) ; KINOSHITA; Kentaro; (Tokyo, JP) ; MORII;
Shigeki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL UNIVERSITY CORPORATION TOTTORI UNIVERSITY
NAGASE & CO., LTD. |
Tottori-shi, Tottori
Osaka-shi, Osaka |
|
JP
JP |
|
|
Assignee: |
NATIONAL UNIVERSITY CORPORATION
TOTTORI UNIVERSITY
Tottori-shi, Tottori
JP
NAGASE & CO., LTD.
Osaka-shi, Osaka
JP
|
Family ID: |
1000005610039 |
Appl. No.: |
16/973301 |
Filed: |
June 11, 2019 |
PCT Filed: |
June 11, 2019 |
PCT NO: |
PCT/JP2019/023143 |
371 Date: |
December 8, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 27/2463 20130101;
H01L 45/1266 20130101; H01L 45/085 20130101; H01L 45/16 20130101;
G11C 13/0011 20130101; H01L 45/146 20130101 |
International
Class: |
H01L 45/00 20060101
H01L045/00; G11C 13/00 20060101 G11C013/00; H01L 27/24 20060101
H01L027/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2018 |
JP |
2018-112008 |
Claims
1. A conductive bridge memory device comprising: a first metal
layer comprising an electrochemically active and easily ionizable
metal; a second metal layer comprising an electrochemically stable
metal; an insulating layer being sandwiched between the first metal
layer and the second metal layer and having a pore communicating
from a first surface being in contact with the first metal layer to
a second surface being in contact with the second metal layer; and
an electrolyte layer being impregnated in the pore of the insulator
layer; wherein the electrolyte layer includes a mixed ionic liquid
in which is mixed a solvate ionic liquid, and a low-viscous ionic
liquid being an ionic liquid having a viscosity coefficient smaller
than that of the solvate ionic liquid.
2. The conductive bridge memory device according to claim 1,
wherein the insulating layer comprising a porous body, and the pore
is an air pore of the porous body.
3. The conductive bridge memory device according to claim 1,
wherein a solvent constituting the solvate ionic liquid is at least
one type of solvent selected from the group consisting of:
##STR00006## (where n is the number of ethyleneoxy groups being 1
or 2; m is the number of methylene groups, which is an integer
being any one of 1 to 3; each of R.sup.1, R.sup.2 can be the same
or different; R.sup.1 denotes an alkyl group whose number of
carbons is between 1 and 6, an alkenyl group whose number of
carbons is between 2 and 6, an alkylnyl group whose number of
carbons is between 2 and 6, a trimethysilyl group, a triethysilyl
group, or a t-butyldimethylsilyl group; R.sup.2 denotes an alkyl
group whose number of carbons is between 1 and 16, an alkenyl group
whose number of carbons is between 2 and 6, an alkylnyl group whose
number of carbons is between 2 and 6, a trimethysilyl group, a
triethysilyl group, or a t-butyldimethylsilyl group; and the
alkenyl group can contain therein an ether functional group, a
thioether functional group.).
4. The conductive bridge memory device according to claim 1,
wherein the low-viscous ionic liquid is at least one type selected
from the group consisting of: ##STR00007## ##STR00008##
##STR00009## ##STR00010## (where R.sup.1 can be the same or
different in the above-mentioned respective chemical formulas, and
denotes an alkyl group whose number of carbons is between 1 and 6,
or an alkenyl group whose number of carbons is between 2 and 6;
R.sup.2 can be the same or different in the above-mentioned
respective chemical formulas, and denotes a hydrogen atom, an alkyl
group whose number of carbons is between 1 and 16, an alkenyl group
whose number of carbons is between 2 and 6, or an alkoxy group. The
alkenyl group can contain therein an ether functional group, a
thioether functional group. R.sup.3 can be the same or different in
the above-mentioned respective chemical formulas, and denotes a
hydrogen atom, a phenyl group, a methyl group, or an isopropyl
group. n in chemical formula (5) denotes the number of methylene
units, where n=1 or 2. In chemical formula (8), R.sup.1 and R.sup.2
can have carbon chains connected mutually, in which case they
denote a trimethylene group, a tetramethylene group, a
pentamethylene group, a hexamethylene group, or a heptamethylene
group. Anion (X) in the ionic liquid can be the same or different
in the above-mentioned respective chemical formulas, and denotes
AlCl.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-,
N(SO.sub.2CF.sub.3).sub.2.sup.-, N(SO.sub.2F).sub.2.sup.-,
N(CN).sub.2.sup.-, MeSO.sub.3.sup.-, MeSO.sub.4.sup.-,
CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-, CF.sub.3COO.sup.-,
RCOO.sup.-, RSO.sub.4.sup.-, RCH(NH.sub.2)COO.sup.-,
SO.sub.4.sup.2-, ClO.sub.4.sup.-, Me.sub.2PO.sub.4.sup.-,
(HF).sub.2.3F.sup.-. (Here, R denotes H, an alkyl group, an
alkyloxy group.)).
5. The conductive bridge memory device according to claim 1,
wherein a:b being the mixing ratio of "a" moles of the solvate
ionic liquid and "b" moles of the low-viscous ionic liquid is 1:(1
to 3).
6. The conductive bridge memory device according to claim 1,
wherein a metal salt or a metal ion of a metal being more difficult
to be oxidized than a metal of the first metal layer is mixed into
the mixed ionic liquid.
7. The conductive bridge memory device according to claim 6,
wherein the metal of the metal salt or the metal ion is selected
from the group consisting of a silver ion, a gold ion, a palladium
ion, a rhodium ion, a ruthenium ion, and a platinum ion.
8. The conductive bridge memory device according to claim 1,
wherein the insulator layer is at least one type selected from the
group consisting of a polycrystalline or amorphous of a metal oxide
or a semiconductor oxide, including alumina, hafnia, silicon oxide,
and a porous body being formed by a self-assembling phenomenon,
including a metal organic framework.
9. The conductive bridge memory device according to claim 1,
wherein the insulating layer having the pore is an amorphous
insulator layer.
10. The conductive bridge memory device according to claim 2,
wherein the porosity of the porous body is greater than or equal to
10% and less than or equal to 80%.
11. The conductive bridge memory device according to claim 1,
wherein the size of the pore of the insulating layer is greater
than or equal to 0.1 nm and less than or equal to 30 nm.
12. A switching device comprising: a first metal layer comprising
an electrochemically active and easily ionizable metal; a second
metal layer comprising an electrochemically stable metal; an
insulating layer being sandwiched between the first metal layer and
the second metal layer and having a pore communicating from a first
surface being in contact with the first metal layer to a second
surface being in contact with the second metal layer; and an
electrolyte layer being impregnated in the pore of the insulator
layer, wherein the electrolyte layer includes a mixed ionic liquid
in which is mixed a solvate ionic liquid, and a low-viscous ionic
liquid being an ionic liquid having a viscosity coefficient smaller
than that of the solvate ionic liquid; wherein
conduction/non-conduction between the first metal layer and the
second metal layer can be controlled in accordance with the
polarity of a voltage applied between the first metal layer and the
second metal layer.
13. A manufacturing method for a conductive bridge memory device,
the manufacturing method comprising: forming an insulating layer on
a surface of a first metal layer comprising an electrochemically
active and easily ionizable metal, in which insulating layer a
first surface is in contact with the surface of the first metal
layer, the insulating layer having a pore communicating between the
first surface and a second surface being opposite to the first
surface; impregnating, in the pore of the insulating layer, an
electrolyte material including a mixed ionic liquid in which is
mixed a solvate ionic liquid, and a low-viscous ionic liquid being
an ionic liquid having a viscosity coefficient smaller than that of
the solvate ionic liquid; and forming, on the second surface of the
insulating layer in which the mixed ionic liquid is impregnated, a
second metal layer comprising an electrochemically stable metal.
Description
TECHNICAL FIELD
[0001] The invention relates to a conductive bridge memory device,
a manufacturing method thereof, and a switching element.
BACKGROUND ART
[0002] CB-RAM (Conducting bridge random access memory) or an atom
switch has a simple structure of an electrode A/a solid electrolyte
(a memory layer)/an electrode B, in which a solid metal oxide
material having nano-sized pores, such as HfO.sub.2, SiO.sub.2,
Al.sub.2O.sub.3, GeSe, or Ag.sub.2S, is sandwiched by the electrode
A, composed of an electrochemically active metal (for example, Ag
or Cu), and the electrode B, composed of an inactive metal (for
example, Pt). By applying a positive voltage to the electrode A
(with respect to the electrode B), atoms constituting the electrode
A are ionized and penetrate into nanopores in the solid electrolyte
and move toward the electrode B. Metal ions reaching the electrode
B receive electrons, and are deposited as a metal. As a result, a
filament-shaped conductive path is formed inside the solid
electrolyte, the filament-shaped conductive path being composed of
the metal constituting the electrode A, and the electrode A and the
electrode B being connected causes a low resistance state to be
realized. On the other hand, applying a negative voltage to the
electrode A (with respect to the electrode B) causes constituent
atoms of the electrode A constituting the filament to be ionized.
The orientation of the electric field is reverse that at the time
of forming the filament, so that atoms constituting the filament
are recovered by the electrode A, causing a high resistance state
to be restored. In other words, the CB-RAM being capable of
replacing the resistance value change by "1" and "0" signals to
function as a memory has excellent features such as high speed,
high integration, and low power consumption, so that this element
is expected as a replacement for a flash memory to face the
minituarization limit in the near future and as a universal memory
having both high speed and non-volatility. The high resistance
state of the CB-RAM can be assumed to be the "OFF" state since
current is difficult to flow therein, while the low resistance
state thereof is assumed to be the "ON" state since current is easy
to flow therein. Thus, the CB-RAM can be used not only as a memory
device but also as a switch, and the conductive path is composed of
a metal, so that it is superior in current transport
characteristics, the possibility thereof for an atom transistor is
expected, and the application thereof to a circuit changeover
switch for FPGA (field programmable gate array) is also
expected.
[0003] FIG. 1 schematically shows a cross section of Cu/a metal
oxide/Pt and a switching process. Here, the metal oxide is set to
be HfO.sub.2 (hafnia). Here, as shown in FIG. 1, the Pt electrode
is grounded, while a voltage is applied to the Cu electrode. A
bipolar operation was confirmed in which a positive bias being
applied to the Cu electrode causes "set" (resistive switching from
high to low resistance) and a negative bias being applied thereto
causes "reset" (resistive switching from low to high resistance).
The function in which the CB-RAM repeats the set-reset resistive
switching is exhibited via a filament forming process called
"forming". While the current-voltage characteristics for the
forming is similar to the current-voltage characteristics for the
set, a forming voltage (V.sub.form) being the voltage at which the
forming occurs is generally higher than the voltage at which the
set occurs (V.sub.set).
[0004] In recent years, various ionic liquids being a liquid molten
salt at room temperature have come to be known. An ionic liquid
being a molten salt exhibits conductivity, and component ions
thereof are tightly bound mutually by the Coulomb force, so that it
exhibits non-volatility and fire resistance. Moreover, in
accordance with the constituent ions thereof, it can be used as
liquid having the function to dissolve various inorganic and
organic compounds (see Non-patent document 1).
[0005] The inventors have revealed that impregnating an ionic
liquid in a metal oxide layer makes it possible to carry out a
design of a stable and high-performance CB-RAM (see Patent document
1, Non-patent documents 1, 2, 3, 4, 5). In a case that an ionic
liquid is impregnated in an HfO.sub.2 (hafnia) layer of a
Cu/HfO.sub.2/Pt cell, stabilization of the hafnia layer in
switching operation improved markedly, and adding therein an ionic
liquid containing 5000 ppm of moisture resulted in a salient
stabilization such that destruction of the hafnia device did not
occur at all even when a voltage of greater than or equal to 10V
was applied thereto (see Non-patent documents 1, 2). Then, they
have studied the design of an ionic liquid to be added and have
revealed that an ionic liquid having a high ionic conductivity and
having an anion with a low proton acceptability can decrease the
set voltage (V.sub.set) and the reset voltage (V.sub.reset) (see
Non-patent document 3). In a case that only an ionic liquid was
added to the hafnia layer of the Cu/HfO2/Pt cell, the V.sub.set,
V.sub.form and V.sub.reset decreased, but no improvement in the
switching endurance was recognized.
[0006] The resistance value change of a CB-RAM is caused by a metal
filament formed in a metal oxide layer. Therefore, in a CB-RAM in
which a metal oxide layer is sandwiched by a Cu foil and a Pt
substrate, a copper filament produced by electrodeposition in the
metal oxide layer is a primary cause for bringing about the
resistance value change. Then, it was found that the switching
endurance improved remarkably when a Copper(II) bis
(trifluoromethyl) sulfonyl (Cu (TFSA).sub.2) solution of
1-Butyl-3-methylimidazolium bis (trifluoromethyl)sulfonylamide
([Bmim][TFSA]) in which Cu.sup.2+ ions are incorporated in an ionic
liquid in advance was added to the hafnia layer (Non-patent
documents 2, 3, 4). While, on the other hand, it was found that
V.sub.set, V.sub.reset increased slightly. It was found that, when
a solvate ionic liquid composed of 2,5,8,11-tetraoxadodecane (G3)
[Cu-G3-(TFSA).sub.2] (see Non-patent document 5) was impregnated in
the hafnia layer to solve this problem, V.sub.set, V.sub.reset
decreased slightly despite [Cu-G3-(TFSA).sub.2] being extremely
high in viscosity and, even more, the switching endurance greatly
improved (see Non-patent document 5).
[0007] "TFSA" is also abbreviated as [Tf.sub.2N], and is also often
denoted, in reagent catalogs and documents, as
"bis(trifluoromethylsulfonyl)imide" ([TFSI]). However, "imide" is
specified in the IUPAC nomenclature as "an amido compound which
connected with two carbonyl group", so that [Tf.sub.2N] being named
as "bis(trifluoromethylsulfonyl)amide" is correct when naming it in
accordance with the IUPAC nomenclature. It can be said that
"(trifluoromethylsulfonyl)imide" is the naming according to the
IUPAC rules. In the specification, [TFSA] will be used in
accordance with the IUPAC nomenclature.
PRIOR ART DOCUMENT
Patent Document
[0008] Patent Document 1: JP 6195155 B1
Non-Patent Document
[0008] [0009] Non-patent document 1: Harada, A.; Yamaoka, H.;
Ogata, R.; Watanabe, K.; Kinoshita, K.; Kishida, S.; Nokami, T.;
Itoh, T. J. Mater. Chem. C, 2015, 3, 6966-6969. [0010] Non-patent
document 2: Harada, A.; Yamaoka, H.; Watanabe, K.; Kinoshita, K.;
Kishida, S.; Fukaya, Y.; Nokami, T.; Itoh, T. Chem. Lett., 2015,
44, 1578-1580. [0011] Non-patent document 3: Harada, A.; Yamaoka,
H.; Tojo, S.; Watanabe, K.; Sakaguchi, A.; Kinoshita, K.; Kishida,
S.; Fukaya, Y.; Matsumoto, K.; Hagiwara, R.; Sakaguchi, H.; Nokami,
T.; Itoh, T. J. Mater. Chem. C, 2016, 4, 7215-7222. [0012]
Non-patent document 4: Kinoshita, K.; Sakaguchi, A.; Harada, A.;
Yamaoka, H.; Kishida, S.; Fukaya, Y.; Nokami, T.; Itoh, T. Jpn. J.
Appl. Phy. 2017, 56, 04CE13. [0013] Non-patent document 5: Yamaoka,
H.; Yamashita, T.; Harada, A.; Sakaguchi, A.; Kinoshita, K.;
Kishida, S.; Hayase, S.; Nokami, T.; Itoh, T. Chem. Lett. 2017, 46,
1832-1835.
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0014] While impregnating a solvate ionic liquid in a hafnia layer
as a metal oxide layer made it possible to achieve a stable and
high-performance CB-RAM structure to an acceptable degree as
described previously, it was found that, in a case of a
Cu/Al.sub.2O.sub.3/Pt cell in which a metal oxide is replaced by
Al.sub.2O.sub.3 (alumina), satisfying both reducing V.sub.set and
V.sub.reset, and improving the switching endurance is not possible
even when any one of an ionic liquid, a copper salt containing
ionic liquid, and a solvate ionic liquid having a copper ion as the
central metal ion [Cu-G3-(Tf.sub.2N).sub.2] is impregnated therein.
Therefore, the invention allows solving such problems in CB-RAM and
to provide a core technology to realize a high-performance
CB-RAM.
[0015] In other words, the invention is to provide a guideline to
realize an optimal ionic liquid design to be added to a metal oxide
layer to develop a high-performance metal oxide CB-RAM being
capable of driving at a low voltage and having a high switching
endurance, and to provide a device configuration to realize the
desired switching characteristics. In particular, it is to provide
a guideline for an ionic liquid design capable of utilizing an
inexpensive metal oxide such as alumina, which inexpensive metal
oxide has not attracted attention conventionally.
Means to Solve the Problem
[0016] A conductive bridge memory device being one embodiment of
the invention includes: a first metal layer including an
electrochemically active and easily ionizable metal; a second metal
layer including an electrochemically stable metal; an insulating
layer being sandwiched between the first metal layer and the second
metal layer and having a pore communicating from a first surface
being in contact with the first metal layer to a second surface
being in contact with the second metal layer; and an electrolyte
layer being impregnated in the pore of the insulator layer, wherein
the electrolyte layer includes a mixed ionic liquid in which is
mixed a solvate ionic liquid, and a low-viscous ionic liquid being
an ionic liquid having a viscosity coefficient smaller than that of
the solvate ionic liquid.
[0017] A switching device being another embodiment of the invention
includes: a first metal layer including an electrochemically active
and easily ionizable metal; a second metal layer including an
electrochemically stable metal; an insulating layer being
sandwiched between the first metal layer and the second metal layer
and having a pore communicating from a first surface being in
contact with the first metal layer to a second surface being in
contact with the second metal layer; and an electrolyte layer being
impregnated in the pore of the insulator layer, wherein the
electrolyte layer includes a mixed ionic liquid in which is mixed a
solvate ionic liquid, and a low-viscous ionic liquid being an ionic
liquid having a viscosity coefficient smaller than that of the
solvate ionic liquid, wherein conduction/non-conduction between the
first metal layer and the second metal layer can be controlled in
accordance with the polarity of a voltage applied between the first
metal layer and the second metal layer.
[0018] A manufacturing method for a conductive bridge memory device
being yet another embodiment of the invention includes: forming an
insulating layer on a surface of a first metal layer including an
electrochemically active and easily ionizable metal, in which
insulating layer a first surface is in contact with the surface of
the first metal layer, the insulating layer having a pore
communicating between the first surface and a second surface being
opposite to the first surface; impregnating, in the pore of the
insulating layer, an electrolyte material including a mixed ionic
liquid in which is mixed a solvate ionic liquid, and a low-viscous
ionic liquid being an ionic liquid having a viscosity coefficient
smaller than that of the solvate ionic liquid; and forming, on the
second surface of the insulating layer in which the mixed ionic
liquid is impregnated, a second metal layer composed of an
electrochemically stable metal.
Effects of the Invention
[0019] According to the invention, adjusting a solvent
intermediating diffusion of atoms of a first metal layer
constituting an electrode A makes it possible to control resistive
switching characteristics, or, in particular, voltage or current
causing resistive switching to occur, allowing to reduce power
required to cause resistive switching to occur and variation in the
switching voltage. This makes it possible to improve switching
endurance. Moreover, there is an advantage that replacing water
being easy to evaporate and to be electrolyzed, the water being
present in an insulator having a pore, such as an oxide
polycrystalline film grain boundary or a porous body, with a
non-volatile and electrochemically unstable mixed ionic liquid
makes it possible to suppress degradation of an insulator layer
such as a metal oxide and make a stable CB-RAM or atom switch.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a view explaining the schematic structure of a
Cu/metal oxide/Pt CB-RAM and the principles of the switching
process.
[0021] FIG. 2 shows a schematic structural view of a
Cu/HfO.sub.2/Pt cell according to one embodiment of the
invention.
[0022] FIG. 3 shows viscosity measurement results at each
temperature according to the type of mixed ionic liquid.
[0023] FIG. 4 shows results of TG measurement (thermal weight
measurement) according to the type of mixed ionic liquid.
[0024] FIG. 5 shows switching endurance testing results of
Cu/Al.sub.2O.sub.3/Pt cell at the time of use in a case that the Cu
concentration of the mixed ionic liquid is changed.
[0025] FIG. 6 shows the cumulative probability of a forming voltage
(V.sub.form) of the Cu/Al.sub.2O.sub.3/Pt cell at the time of
adding the mixed ionic liquid in a case that the mixing ratio of
the mixed ionic liquid is changed.
[0026] FIG. 7 shows the cumulative probability distribution of a
set voltage (V.sub.set) of the Cu/Al.sub.2O.sub.3/Pt cell at the
time of adding the mixed ionic liquid in a case that the mixing
ratio of the mixed ionic liquid is changed.
[0027] FIG. 8 shows the viscosity at each temperature of
Cu-containing DME and so on and G2 solvate ionic liquids.
[0028] FIG. 9 shows results of a TG measurement with respect to the
temperature in a case of changing the mixing ratio of the
Cu-containing DME and the G2 solvate ionic liquids.
[0029] FIG. 10 shows the forming voltage (V.sub.form) distribution
of the Cu/Al.sub.2O.sub.3/Pt cell at the time of adding in a case
of changing the mixing ratio of the Cu-containing DME and the G2
solvate ionic liquids.
[0030] FIG. 11 shows the cumulative probability distribution of the
set voltage (V.sub.set) of the Cu/Al.sub.2O.sub.3/Pt cell in a case
of changing the type or the ratio of the Cu-containing DME and the
G2 solvate ionic liquids.
[0031] FIG. 12 shows the switching endurance of a
Cu/Al.sub.2O.sub.3/Pt element in a case of changing the type or the
mixing ratio of the Cu-containing DME and the G2 solvate ionic
liquids.
[0032] FIG. 13 shows the distribution of the set voltage
(V.sub.set) of the Cu/Al.sub.2O.sub.3/Pt cell in a case of changing
the Ag amount of Ag [TFSA]-containing [Bmim][TFSA].
[0033] FIG. 14 shows the switching endurance of the
Cu/Al.sub.2O.sub.3/Pt element with Ag [TFSA]-containing
[Bmim][TFSA].
[0034] FIG. 15 shows a circuit diagram of an exemplary
configuration of a memory device according to one embodiment of the
invention.
[0035] FIG. 16 shows cross-sectional views of a manufacturing
process for the memory device according to one embodiment of the
invention.
EMBODIMENT FOR CARRYING OUT THE INVENTION
First Embodiment
[0036] A conductive bridge memory device according to a first
embodiment of the invention will be described with reference to
FIG. 2. A memory cell 10 of the conductive bridge memory device
according to the embodiment, as shown with the structure of one
example thereof in FIG. 2, comprises: a first metal layer 1 (an
electrode A) including an electrochemically active and easily
ionizable metal; a second metal layer 2 (an electrode B) including
an electrochemically stable metal; an insulating layer 3 being
sandwiched between the first metal layer 1 and the second metal
layer 2 and having a pore communicating between a first surface 3b
being in contact with the first metal layer 1 and a second surface
3c being in contact with the second metal layer 2; and an
electrolyte layer 4 being impregnated in the pore 3a of the
insulator layer 3. Then, the electrolyte layer 4 contains a mixed
ionic liquid in which is mixed a solvate ionic liquid, and a
low-viscous ionic liquid being an ionic liquid having a viscosity
coefficient smaller than that of the solvate ionic liquid. To make
the explanations easy to understand, FIG. 2 shows, as the insulator
layer 3, an example of the pore 3a portion and the insulator layer
3 being divided by a porous body of hafnia (HfO.sub.2), in which
porous body the pore 3a tends to have a columnar shape. However, as
described below, the insulator layer 3 including a porous body of
alumina, in which porous body the pores 3a are mutually connected
in a complex shape, also act in the same manner. It suffices that
the pore 3a of the insulator layer 3 communicates between the first
surface 3b being in contact with the first metal layer 1 and the
second surface 3c being in contact with the second metal layer 2.
In the specification, the term "communicate" means being mutually
connected to such a degree that at least liquid can pass through
regardless of the shape of the pore. In other words, the pores
continue such that liquid can pass therethrough between the first
surface 3b and the second surface 3c.
[0037] Here, "solvate" means the state in which molecules of a
solvent surround the periphery of molecules or ions of a solute in
a solution to create a group of molecules. The solvate ionic liquid
means such an ionic liquid having solvate. Moreover, the term "a
low-viscous ionic liquid" is a name to be referred to for the sake
of convenience in the specification, and means an ionic liquid
having a smaller viscosity (viscosity coefficient) than that of the
solvate ionic liquid. Furthermore, the term "a mixed ionic liquid"
also being a name to be referred to for the sake of convenience in
the specification means an ionic liquid in which is mixed the
solvate ionic liquid and the above-described low-viscous ionic
liquid.
[0038] As described previously, the inventors have made an arduous
study in quest for performance improvement of the conductive bridge
memory device (CBRAM). As a result, impregnating an ionic liquid as
the electrolyte layer 4 to be impregnated in the pore 3a of the
insulating layer 3 including a porous body have made it possible to
switch between on and off at high speed. However, there is a
problem that the operating voltages such as the set voltage
(V.sub.set) increase, and, as a result of an arduous study by the
inventors, it was found that the cause thereof is that a Cu.sup.2+
ion covers the surface of the second metal layer 2 and the
following Cu.sup.2+ ion is difficult to reach the second metal
layer 2, and that there is a need to increase the applying voltage.
Then, it was found that, as a countermeasure therefor, making the
electrolyte to be a solvate ionic liquid in which a Cu.sup.2+ ion
is surrounded by a solvent made it possible to substantially
decrease the operating voltages such as the set voltage
(V.sub.set). However, a problem occurred that, even when such a
solvate ionic liquid was used, in a case of using an inexpensive
oxide layer such as alumina (Al.sub.2O.sub.3) as the insulator
layer 3, the switching speed decreased and the switching endurance
decreased. Thus, the inventors made a further arduous study to
solve this problem.
[0039] In other words, it was found the cause thereof lies in that,
when using the solvate ionic liquid as the electrolyte layer 4, the
viscosity of the solvate ionic liquid is large and the speed of the
Cu.sup.2+ ion decreases in the pore 3a being formed in a complex
shape, such as in alumina, due to attraction by the Coulomb force.
Then, impregnating, in the pore 3a of the insulator layer 3, as a
mixed ionic liquid in which is mixed, into a solvate ionic liquid,
a low-viscous ionic liquid being a so-called normal ionic liquid,
the low-viscous ionic liquid having the viscosity lower than that
of the solvate ionic liquid, made it possible to obtain a
conductive bridge memory device having a low operating voltage and
having an excellent switching endurance.
[0040] As a solvent of the solvate ionic liquid to be used as the
electrolyte layer 4 according to the embodiment, at least one type
of solvent is used, the at least one type of solvent to be selected
from the group consisting of:
##STR00001##
(where n is the number of ethyleneoxy groups being 1 or 2; m is the
number of methylene groups, which is an integer being any one of 1
to 3; each of R.sup.1, R.sup.2 can be the same or different;
R.sup.1 denotes an alkyl group whose number of carbons is between 1
and 6, an alkenyl group whose number of carbons is between 2 and 6,
an alkylnyl group whose number of carbons is between 2 and 6, a
trimethysilyl group, a triethysilyl group, or a
t-butyldimethylsilyl group; R.sup.2 denotes an alkyl group whose
number of carbons is between 1 and 16, an alkenyl group whose
number of carbons is between 2 and 6, an alkylnyl group whose
number of carbons is between 2 and 6, a trimethysilyl group, a
triethysilyl group, or a t-butyldimethylsilyl group; and the
alkenyl group can contain therein an ether functional group, a
thioether functional group.)
[0041] While a metal ion being a solute constituting the solvate
ionic liquid is desirably a copper ion that can be a filament
component, it is not necessary to stick to a component metal
constituting a filament (a metal of the first metal layer 1). For
example, a metal ion such as a precious metal ion species, for
example, a silver ion, a gold ion, a palladium ion, a rhodium ion,
a ruthenium ion, or a platinum ion, or cobalt, nickel, or a
lanthanoid metal ion such as Europium (Eu) can be utilized.
Moreover, a plurality of these metal ions can be mixed. In other
words, even in a case that copper is used as a material for the
metal layer 1 and the filament is formed of copper, the
above-mentioned various metal ions can be used as a solute of the
solvate ionic liquid, or a copper ion and these metal ions can be
mixed. The ratio of the metal ions of the solvate ionic liquid with
respect to the overall metal constituting the filament is very
small.
[0042] While a counter anion constituting the solvate ionic liquid
is desirably bis(trifluoromethylsulfonyl) amide
(N(SO.sub.2CF.sub.3).sub.2.sup.-:TFSA), bis(fluorosulfonyl)amide
(N(SO.sub.2F).sub.2.sup.-:FSA), it suffices to be an anion species
to be liquid when solvated, and the other ones include
AlCl.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-,
MeSO.sub.3.sup.-, CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-,
CF.sub.3COO.sup.-, RCOO.sup.-, RSO.sub.4.sup.-,
RCH(NH.sub.2)COO.sup.-, SO.sub.4.sup.2-, ClO.sub.4.sup.-,
(HF).sub.2.3F. (Here, R denotes H, an alkyl group, or an alkyloxy
group).
[0043] Moreover, as the low-viscous ionic liquid, for example, the
at least one type to be selected from the group consisting of:
##STR00002## ##STR00003##
can be used. (where R.sup.1 can be the same or different in the
above-mentioned respective chemical formulas, and denotes an alkyl
group whose number of carbons is between 1 and 6, or an alkenyl
group whose number of carbons is between 2 and 6; R.sup.2 can be
the same or different in the above-mentioned respective chemical
formulas, and denotes a hydrogen atom, an alkyl group whose number
of carbons is between 1 and 16, an alkenyl group whose number of
carbons is between 2 and 6, or an alkoxy group. The alkenyl group
can contain therein an ether functional group, a thioether
functional group. R.sup.3 can be the same or different in the
above-mentioned respective chemical formulas, and denotes a
hydrogen atom, a phenyl group, a methyl group, or an isopropyl
group. n in chemical formula (5) denotes the number of methylene
units, where n=1 or 2. In chemical formula (8), R.sup.1 and R.sup.2
can have carbon chains connected mutually, in which case they
denote a trimethylene group, a tetramethylene group, a
pentamethylene group, a hexamethylene group, or a heptamethylene
group. Anion (X) in the ionic liquid can be the same or different
in the above-mentioned respective chemical formulas, and denotes
AlCl.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-,
N(SO.sub.2CF.sub.3).sub.2.sup.-, N(SO.sub.2F).sub.2.sup.-,
N(CN).sub.2.sup.-, MeSO.sub.3.sup.-, MeSO.sub.4.sup.-,
CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-, CF.sub.3COO.sup.-,
RCOO.sup.-, RSO.sub.4.sup.-, RCH(NH.sub.2)COO.sup.-,
SO.sub.4.sup.2-, ClO.sub.4.sup.-, Me.sub.2PO.sub.4.sup.-,
(HF).sub.2.3F.sup.-. (Here, R denotes H, an alkyl group, or an
alkyloxy group.)
[0044] As a further specific example of a cation and an anion
constituting the low-viscous ionic liquid,
##STR00004## ##STR00005##
can be used.
[0045] The above-mentioned solvate ionic liquid or the low-viscous
ionic liquid is not limited to the one type in the above-mentioned
example, but can be a mixture of a plurality of types in the
above-mentioned example. Moreover, while the mixing ratio (the
molar ratio) of the solvate ionic liquid and the low-viscous ionic
liquid is adjusted as needed in accordance with such as the type of
the pore 3a of the insulating layer (metal oxide layer) 3 to be
used, as a preferable range, for example, the ratio of (the solvate
ionic liquid):(the low-viscous ionic liquid) is 1:(1 to 3).
[0046] Moreover, as a result of the inventors having made a further
arduous study, it was found that incorporating, in the solvate
ionic liquid, or a low-viscous ionic liquid, or a mixed ionic
liquid in which these are mixed, a metal salt to be dissolved
therein makes it possible to further improve the CB-RAM
function.
[0047] It is not necessary to stick to a metal constituting a
filament, or, in other words, a metal of the first metal layer 1,
as a cation of a metal salt to be dissolved in the mixed ionic
liquid, the metal salt to improve the CB-RAM function as described
in the above, so that the metal salt can be a metal salt capable of
being dissolved in the low-viscous ionic liquid or the solvate
ionic liquid. In this case, a metal having a smaller ionization
tendency than that of the metal of the first metal layer 1 is
desirable. In other words, in a case that copper is used as the
metal layer 1, a silver salt, a gold salt, a palladium salt, a
rhodium salt, a ruthenium salt, or a platinum salt is possible, for
example. Adding the silver salt in particular makes it possible to
substantially improve the CB-RAM function. Moreover, this metal
salt can be not only a mono salt, but also a double salt.
[0048] While, as an anion of a metal salt to improve the CB-RAM
function as described in the above and to be dissolved in the mixed
ionic liquid, bis(trifluoromethylsulfonyl)amide
(N(SO.sub.2CF.sub.3).sub.2.sup.-:TFSA), bis(fluorosulfonyl)amide
(N(SO.sub.2F).sub.2.sup.-:FSA) are desirable, it suffices that it
be an anion species to be liquid when solvating with a metal ion,
and the other ones include AlCl.sub.4.sup.-, BF.sub.4.sup.-,
PF.sub.6.sup.-, SbF.sub.6.sup.-, MeSO.sub.3.sup.-,
CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-, CF.sub.3COO.sup.-,
RCOO.sup.-, RSO.sub.4.sup.-, RCH(NH.sub.2)COO.sup.-,
SO.sub.4.sup.2-, ClO.sub.4.sup.-, (HF).sub.2.3F (Here, R denotes H,
an alkyl group, or an alkyloxy group). Moreover, a plurality of
these anions can be mixed.
[0049] The low-viscous ionic liquid or the solvate ionic liquid
containing the metal salt or the metal ion to improve the CB-RAM
function can be a single mixed ionic liquid or a plurality of mixed
ionic liquids, or a solvate ionic liquid containing different types
of metal ions, so that there is a need to adjust and optimize the
mixing ratio in accordance with the metal oxide layer to be
used.
[0050] Examples of the types of mixing between the solvate ionic
liquid and the low-viscous ionic liquid and the mixing ratio
thereof will be described later in the specific examples.
[0051] As described previously, the structure of the CB-RAM
according to the embodiment is configured to be the structure in
which the insulator layer 3 having the pore 3a to communicate
between the first surface 3b and the second surface 3c is
sandwiched between the first metal layer 1 and the second metal
layer 2. As the first metal layer 1, an electrochemically active
and easily ionizable metal is used. Specifically, a metal such as
Cu, Ag, Ti, Zn, V, or an alloy of these metals can be cited.
Moreover, as a metal for the second metal layer 2, an
electrochemically stable metal is used. Specifically, a metal such
as Pt, Au, Ir, Ru, Rh, or an alloy of these metals can be
cited.
[0052] For the insulator layer 3 having the pore 3a, a
polycrystalline or amorphous of a solid metal oxide or a
semiconductor oxide having a nano-sized pore, such as alumina,
hafnia, GeSe, or Ag.sub.2S, and a porous body being formed by a
self-assembling phenomenon, such as a metal organic framework, can
be cited. As described previously, the example shown in FIG. 2 is
an example of hafnia (HfO.sub.2), the example being an example in
which the pore 3a is formed in a columnar shape. While the pore 3a
is not formed in the columnar shape in a case of the porous body
being alumina (Al.sub.2O.sub.3), it suffices that the pores 3a be
mutually connected between the first surface 3b and the second
surface 3c and the pores 3a in which filaments continue be formed
between the first metal layer 1 and the second metal layer 2. In
other words, it suffices that the pore 3a be communicated between
the first surface 3b and the second surface 3c, so that the
insulator layer 3 does not have to be a porous body. Therefore,
alumina is preferable in that it is inexpensive and easy to obtain,
and it suffices that the insulator layer 3 have mutually connecting
pores 3a. For example, the insulator layer 3 can also be an
amorphous insulator. In the example shown in FIG. 2, the diameter d
of the pore 3a is approximately between 2 to 5 nm, while the
thickness t of the porous body is approximately between 2 to 50 nm.
While the pore 3a is not formed in a columnar shape even in a case
of alumina (Al.sub.2O.sub.3), the size thereof is approximately the
same as in a case of hafnia.
[0053] The size of the pore 3a of the porous body is, preferably,
greater than or equal to 0.1 nm and less than or equal to 30 nm,
and the porosity thereof is, preferably, greater than or equal to
10% and less than or equal to 80%. The previously-described liquid
electrolyte material is impregnated in the pore 3a of the insulator
layer 3. As methods of impregnating this electrolyte material in
the pore 3a of the insulator layer 3, for example, an electrolyte
material can be applied to the surface of the insulator layer 3
having the pore 3a to impregnate the applied electrolyte material
thereto using a capillary phenomenon, an electrolyte material can
be dropped onto the surface of the insulator layer 3 to vacuum
suction the dropped electrolyte material from the rear surface, or
the insulator layer 3 having the pore 3a is dipped in an
electrolyte material liquid to cause the dipped insulator layer 3
to be impregnated therein.
[0054] As described previously, adding an electrolyte material in
the insulator layer 3 (a metal oxide layer) makes it possible to
improve the CB-RAM function such as the forming voltage
(V.sub.form), the set voltage (V.sub.set); the reset voltage
(V.sub.reset), and the switching endurance, and, as the
above-mentioned electrolyte material, a metal salt-containing ionic
liquid can be selected. While a metal salt was dissolved in a
single ionic liquid (either a low-viscous ionic liquid or a solvate
ionic liquid) to realize an improvement in the CB-RAM function in
the prior art Patent document 1 and studies (Non-patent documents 1
to 5), the invention was completed by making a metal
salt-containing ionic liquid to improve the CB-RAM function using a
technique to mix a plurality of ionic liquids being a solvate ionic
liquid and a low-viscous ionic liquid and to optimize the mixing
ratio thereof. Next, further explanations will be given using
specific examples.
[0055] [Making of Element]
[0056] A CB-RAM element was made by impregnating, in the pore 3a of
an Al.sub.2O.sub.3 film having a thickness of 20 nm (a porous
body), a mixed ionic liquid in which is mixed Cu(TFSA).sub.2 and
Triglyme(G3) as a solvate ionic liquid and [Bmim][TFSA] as a
low-viscous ionic liquid (written below as
Cu(TFSA).sub.2-Triglyme(G3)-[Bmim][TFSA]). Specifically, the
Al.sub.2O.sub.3 film was formed on a Pt thin film (the second metal
layer 2) by sputtering, and 1 .mu.l (microliter) each of the
solvate ionic liquid and the low-viscous ionic liquid was dropped
thereon, and a Cu-probe was brought into contact with an
Al.sub.2O.sub.3 thin film through this ionic liquid layer instead
of the first metal layer 1 in FIG. 2 to form a Cu/HfO.sub.2/Pt
cell. The forming voltage (V.sub.form), the set voltage
(V.sub.set); the reset voltage (V.sub.reset), and the switching
endurance were measured using this element. In this experiment, to
avoid the effect of water in the atmosphere, a dry N.sub.2 gas was
supplied into a measurement container from 15 minutes prior to
measurement, and a resistance change at the time of applying
voltage was investigated in the dry N.sub.2 gas atmosphere. At
least 20 points were measured for one cell.
[0057] [Viscosity, Conductivity of Mixed Ionic Liquid]
[0058] When measurements were carried out for
Cu(TFSA).sub.2:G3:[Bmim][TFSA]=1:1:0.5, 1:1:1, 1:1:2, 1:1:3 (molar
ratio) samples, as shown in FIG. 3, as the amount of [Bmim][TFSA]
in the mixed ionic liquid increased, the conductivity improved with
the decrease in the viscosity. It was revealed that the viscosity
decreased and the conductivity increased by mixing of the solvate
ionic liquid and the ionic liquid [Bmim][TFSA] (see Table 1), and
it was revealed that the viscosity and the conductivity could be
controlled by mixing of the solvate ionic liquid and an imidazolium
salt ionic liquid.
TABLE-US-00001 TABLE 1 VISCOSITY/CONDUCTIVITY OF Cu-CONTAINING
MIXED ION LIQUIDS Cu(TFSA).sub.2:G3:[Bmim] Cu(TFSA).sub.2:G3 =
Cu(TFSA).sub.2:G3 = [TFSA] mixing ratio 1:1 1:2 1:1:0.5 1:1:1 1:1:2
1:1:3 VISCOSITY 3023 470.1 284.8 165.0 111.7 80.27 (cP)/35.degree.
C. CONDUCTIVITY 0.080 0.48 0.74 1.0 1.6 2.3 (mS/cm)/30.degree.
C.
[0059] [Thermal Stability Measurement of Mixed Ionic Liquid]
[0060] Results of TG measurement of mixed ionic liquids in each of
which a low-viscous ionic liquid (an imidazolium salt ionic liquid
ionic liquid) is mixed in a solvate ionic liquid, by comparison
with Cu(TFSA).sub.2-G3 ionic liquids, are shown in FIG. 4. It was
revealed that, while a large weight decrease occurred in the
vicinity of 230 to 240 degrees C. in the mixed ionic liquids, all
exhibited improvement in thermal stability with respect to G3 alone
and Cu(TFSA).sub.2 alone.
[0061] [Switching Endurance Testing Results at the Time of Adding
Mixed Ionic Liquid to Cu/Al.sub.2O.sub.3/Pt Element]
[0062] The switching endurance is shown in FIG. 5. All exhibited
improvement with respect to Blank or Cu(TFSA).sub.2:G3=1:1.
Comparing in accordance with the type of mixed ionic liquid, the
Cu(TFSA).sub.2:G3:[Bmim][TFSA]=1:1:2 exhibited the highest
operating rate. The difference in the switching endurance is
believed to be affected by the Cu concentration in the samples and
the viscosity of the samples. First, with respect to the Cu
concentration, as seen in the case of the Cu ion-dissolved ionic
liquid or the Cu(TFSA).sub.2-G3 solvate ionic liquid in the hafnia
device, securing a sufficient Cu concentration to suppress
segregation of a filament is believed to be a factor in stabilizing
rupture-formation of the filament. While the switching endurance
improved with an increase in the Cu concentration in the hafnia
element (see Non-patent documents 3, 4, 5), it only improved
slightly in the alumina element. It is well known that, when
comparing alumina and hafnia, alumina is more likely to be
amorphous at the time of film formation, and a crystalline grain
boundary is more difficult to be formed in alumina. In 1:1:0.5 and
1:1:1 having a higher Cu concentration with respect to 1:1:2
exhibiting the highest operating rate, it is believed that
formation-rupture of the filament unstabilized due to the high
viscosity and the difficulty of entry into a nanopore being formed
in the alumina. On the other hand, it is believed that, while
penetrating into a pore in the alumina is easy as the viscosity
decreased in 1:1:3, the Cu concentration being the essence is low,
thus limiting the impact on the formation-rupture process
stabilization of the filament.
[0063] [V.sub.form Measurement Experiment at the Time of Adding
Mixed Ionic Liquid to Cu/Al.sub.2O.sub.3/Pt Element]
[0064] A forming voltage at the time of adding the mixed ionic
liquid is shown in FIG. 6, while a set voltage is shown in FIG. 7.
Both the forming voltage and the set voltage substantially
improved. With respect to the forming voltage, 1:1:1 and 1:1:3
exhibited particularly superior results. These results are believed
to be affected by the solution resistance (Rsol) and the charge
transfer resistance (Rct). Rsol tends to decrease and Rct tends to
increase as the amount of [Bmim][TFSA] in the mixed ionic liquid
increases. It is believed that, with respect to the mixing ratio
1:1:3, Rsol is very low, so that the forming voltage decreases to
compensate for the increase in Rct, while, with respect to the
mixing ratio 1:1:1, values of Rsol and Rct adapted to the decrease
in the forming voltage. Moreover, an excessive set voltage being
observed in the case of an ionic liquid not being added (Blank)
dramatically decreased with adding of a mixed ionic liquid. In this
way, it was found that the operating voltage of an Al.sub.2O.sub.3
element could be improved substantially by using a mixed ionic
liquid.
[0065] [Type of Solvate Ionic Liquid]
[0066] It is believed that while G3 (n=2 in Chemical formula 1) has
been used conventionally as an ether compound to make a solvate
ionic liquid (see Non-patent documents 5, 6, 7), besides G3,
various ethers capable forming a complex with Cu ions can be
utilized. For example, (Diglyme: G2) being denoted with n=1 in
Chemical formula 1 and dimethoxyethane (DME) being denoted with n=1
in Chemical formula 2 are also expected to be utilized. Thus, the
demonstration experiments thereof were carried out.
[0067] [Viscosity Measurement of Solvate Ionic Liquids Using DME,
G2]
[0068] Viscosity measurement results of solvate ionic liquids using
DME, G2 are shown in FIG. 8 while comparison of the viscosity and
the conductivity are shown in Table 2. The viscosity decreased by
using DME and G2 having a shorter side chain than that of G3, and
the conductivity improved in conjunction therewith.
TABLE-US-00002 TABLE 2 VISCOSITY/CONDUCTIVITY OF Cu-CONTAINING DME,
G2 SOLVATED ION LIQUIDS [Bmim] Cu:G3 Cu:G3 Cu:DME Cu:DME Cu:G2
[TFSA] 1:1 1:2 1:1 1:2 1:2 VISCOSITY 32.8 3023 470.1 165.9* 155.2
241.2 (cP)/35.degree. C. CONDUCTIVITY 4.7 0.08 0.48 0.26 1.2 0.81
(mS cm.sup.-1/30.degree. C.
[0069] [Thermal Stability Measurement Experiment of Cu-Containing
DME, G2 Solvate Ionic Liquids]
[0070] FIG. 9 shows TG measurement results of the Cu-containing
DME, G2 solvate ionic liquids. Both Cu(TFSA).sub.2-DME and
Cu(TFSA).sub.2-G2 improved in thermal stability with respect to
Cu(TFSA).sub.2 alone or G3 alone.
[0071] [Forming and Set Voltages of Cu/Al.sub.2O.sub.3/Pt CB-RAM at
the Time of Adding Cu-Containing DME, G2 Solvate Ionic Liquids]
[0072] The forming voltage distribution of the
Cu/Al.sub.2O.sub.3/Pt cell at the time of adding the Cu-containing
DME, the G2 solvate ionic liquids is shown in FIG. 10, while the
set voltage distribution thereof is shown in FIG. 11. Both
decreased when a solvate ionic liquid was added to the alumina
layer. With respect to the forming voltage, Cu(TFSA).sub.2:DME=1:2
was particularly superior. This is believed to be caused by the
fact that it has the highest conductivity of the solvate ionic
liquids used. With respect to the set voltage, as shown in FIG. 11,
using the solvate ionic liquid caused the set voltage thereof to be
shifted overall to the low voltage side. It was found, from these
results, that using the G2, DME solvate ionic liquids makes it
possible to decrease the operating voltage.
[0073] [Switching Endurance of Cu/Al.sub.2O.sub.3/Pt Device to
which is Added Cu-Containing DME, G2 Solvate Ionic Liquids]
[0074] The switching endurance is shown in FIG. 12.
Cu(TFSA).sub.2-DME solvate ionic liquid improved in particular.
Comparing among the Cu(TFSA).sub.2-G3 solvate ionic liquids,
Cu(TFSA).sub.2:G3=1:2 having a lower viscosity and a higher
conductivity exhibited the highest operating rate and, even
comparing among the Cu(TFSA).sub.2-DME solvate ionic liquids,
Cu(TFSA).sub.2:DME=1:2 similarly exhibited the higher operating
rate. This shows that facility of movement of copper ions is more
effective in the CB-RAM switching resistance in the Al.sub.2O.sub.3
element in which a clear crystalline grain boundary is difficult to
be formed. The Cu(TFSA).sub.2-DME solvate ionic liquid has a
smaller ion pair size with respect to that of the Cu(TFSA).sub.2-G3
solvate ionic liquid. Therefore, besides the viscosity, the size of
the solvate ionic liquid could also be related as a reason that the
Cu(TFSA).sub.2-DME solvate ionic liquid exhibited superior results
in the switching endurance. It is believed that the movement of a
solvate ionic liquid having a small ion pair size be advantageous
at a complex crystalline grain boundary as in the Al.sub.2O.sub.3
element and rupture-formation of a filament brought about
stabilization.
[0075] [Set Voltage (V.sub.set) in Case of Adding Ag
[TFSA]-Containing [Bmim][TFSA] to Cu/Al.sub.2O.sub.3/Pt Cell]
[0076] As a metal salt to be dissolved in a metal salt-containing
ionic liquid to improve the CB-RAM function, there is no need to
stick to a metal constituting a filament; the above-mentioned metal
salt can be a metal salt capable of being dissolved in an ionic
liquid or a solvate ionic liquid, so that it is possibly a salt of
a metal being more precious than copper, for example, a silver
salt, a gold salt, a palladium salt, a rhodium salt, a ruthenium
salt, or a platinum salt. Adding a silver salt
(Ag(TFSA))-containing low-viscous ionic liquid [Bmim][TFSA] to a
Cu/Al.sub.2O.sub.3/Pt device to demonstrate this (FIG. 13) causes
the set voltage (V.sub.set) to decrease, and no abnormal voltage
was observed. While this is a model experiment and the mixed ionic
liquid-type is not measured, similar results are believed to be
obtained by adding Ag (TFSA) to a mixed ionic liquid.
[0077] [Switching Endurance of Cu/Al.sub.2O.sub.3/Pt Element with
Ag[TFSA]-Containing [Bmim][TFSA]]
[0078] FIG. 14 shows results of investigating the switching
endurance with an Ag [TFSA]-containing low-viscous ionic liquid
[Bmim][TFSA] being added in the Cu/Al.sub.2O.sub.3/Pt element. As
shown in this figure, the switching endurance of the
Cu/Al.sub.2O.sub.3/Pt device greatly improved. While this is a
model experiment and the mixed ionic liquid-type is not measured,
similar results are believed to be obtained by adding Ag (TFSA) to
a mixed ionic liquid.
[0079] [Configuration of Memory Device]
[0080] A normal memory device is configured by memory cells 10
(CB-RAM devices) being arranged in a matrix, the memory cells 10
being formed as described in the above and shown in FIG. 2, a
selection transistor 11 being connected to each of the cells, and a
word line WL being connected to a gate G of the above-mentioned
selection transistor 11, a bit line BL being connected to a drain D
thereof, and a source line SL (a signal line) being connected, via
a memory cell 10, to a source S thereof, as shown in an equivalent
circuit diagram in FIG. 15. Then, each of the memory cells 10 in a
matrix being selected in accordance with desired data makes it
possible to carry out displaying a desired image or storing in a
predetermined location. In FIG. 15, the letter 12 denotes a bit
line selection transistor.
[0081] Here, the CB-RAM device 10 according to the embodiment is a
non-volatile storage device capable of exhibiting a reversible
change in electrical resistance by an electrical stress, for
example, applying a direct current or a pulse voltage, and storing
information in correspondence with a resistance by the resistance
being held even when power is turned off, so that, for example, as
a CB-RAM device, the structure in which Al.sub.2O.sub.3, SiO.sub.2,
NiO.sub.y(y=1), TiO.sub.z, HfO.sub.z(z=2) or the like being formed
using such as sol-gel, sputtering, or MOCVD method is sandwiched
between an active electrode (an electrode A) and an inactive
electrode (an electrode B) is made.
[0082] While the previously-described configuration of the memory
cell array can be set to be the same as the configuration shown in
JP 4684297 B1, JP4662990 B1, or JP6108559 B1, in a memory device (a
memory cell array) according to the embodiment, an insulator layer
including an oxide layer having a mesopore or a nanopore in a
memory layer of a CB-RAM device being used for the memory layer, a
mixed ionic liquid being impregnated in such a pore, and absorbing
and holding a solvent that can affect the electrochemical action
and the movement of a metal ion is different from the conventional
structure. According to the embodiment, absorbing and holding a
mixed ionic liquid is different from the conventional one. It is
preferable to contain a different metal salt or metal ion in this
mixed ionic liquid.
[0083] [Manufacturing Method for Memory Device]
[0084] FIG. 16 shows manufacturing steps for a non-volatile memory
device according to the embodiment. In the manufacturing method for
the memory device according to the embodiment, first, as shown in
(a) in FIG. 16, a transistor 11 having a source S, a drain D, and a
gate G is formed on a surface of a semiconductor substrate 21 using
a normal semiconductor process. Then, thereon an interlayer
insulating film 24 is formed and a contact plug 25 is formed, and
each of a source line SL on the contact plug 25 being connected to
the source S and a second metal layer (an electrode B) 2 on the
contact plug 25 being connected to the drain D is formed. For the
second metal layer 2, for example, Pt was formed to a thickness of
100 nm under the conditions of the atmospheric gas of
Ar:O.sub.2=1:0, the total pressure of 0.7 Pa, and room temperature.
In (a) in FIG. 15, the letter 22 denotes a gate insulating
film.
[0085] Then, as shown in (b) in FIG. 16, an interlayer insulating
film 26 being composed of such as SiO.sub.2 (an Si thermal oxide
film), for example, is formed between the source line SL and the
second metal layer 2. Thereafter, as shown in (c) in FIG. 16, an
insulator layer 3 being composed of a porous body such as hafnia or
alumina, the porous body having a mesopore or a nanopore, for
example, is formed. Specifically, with the substrate temperature at
approximately 300 degrees C., the total pressure at 5. 3 Pa, and
Ar:O.sub.2 at the ratio of 3.8:1.5, HfO.sub.2 is formed to 20 nm,
for example, on the electrode B (for example, Pt) using reactive RF
magnetron sputtering method. Here, it is important that the
HfO.sub.2 thin film undergoes a polycrystalline growth and a
crystalline grain boundary is introduced into the thin film.
[0086] Thereafter, as shown in (d) in FIG. 16, the mixed ionic
liquid 4 as described previously is applied to the insulator layer
3 composed of the porous body by the mixed ionic liquid 4 being
dropped thereon. Specifically, for example, [Bmim], [TFSA] being a
low-viscous ionic liquid and Cu(TFSA).sub.2 and Triglyme(G3) being
a solvate ionic liquid are uniformly applied, by spin coating, on
the insulator layer 3 composed of the porous body as described
previously, causing the applied liquid to be absorbed in a
HfO.sub.2 thin film grain boundary crystalline grain of the porous
body functioning as a mesopore or a nanopore in the atmosphere or
under low pressure. In particular, under low pressure, replacement
between the moisture being capillary condensed in the crystalline
grain boundary and the mixed ionic liquid is carried out
efficiently.
[0087] Then, as shown in (e) in FIG. 16, a first metal layer (an
electrode A) 1 is formed using such as copper and patterned to
cause each of the memory cells 10 to be formed. With respect to
forming of the first metal layer, specifically, a Cu film is formed
to a thickness of 100 nm under the conditions of the atmospheric
gas of Ar:O.sub.2=1:0, the total pressure of 0.7 Pa, and room
temperature. Next, as shown in (f) in FIG. 16, an interlayer
insulating film 27 being composed of such as SiO.sub.2 is formed so
as to cover the surrounding of each of the cells 10 therewith.
Then, the memory device according to the embodiment is completed by
a contact plug 28 connected to the first metal layer 1 being formed
in the interlayer insulating film 27 and a bit line BL being formed
on the surface thereof so as to connect each contact plug 28.
[0088] A cross-sectional view of the Cu/HfO.sub.2/Pt structure
being made by forming Pt (the electrode B) on the interlayer
insulating film 26 composed of SiO.sub.2 as described in the above,
and then an HfO.sub.2 thin film (a porous body) and a film of Cu
(the electrode A) is shown in FIG. 2. It can be seen that HfO.sub.2
grows in a columnar shape and a nano-sized gap (grain boundary) is
present in between columnar-shaped crystals.
[0089] While not shown in FIG. 16, the gate line GL (see FIG. 15)
to connect the gate G of each of the cells and the source line SL
(see FIG. 15) to connect the source S of each of the cells are
formed so as to extend in the perpendicular direction to the paper
surface and connected as shown in FIG. 15.
[0090] [Operation of Storage Device]
[0091] Next, an operation at the time of set of a memory cell array
(a storage device) according to the embodiment is described using
FIG. 15. As described previously, set is a writing process from
high resistance to low resistance. First, a selection transistor 12
is turned on, the selection transistor 12 being connected to a bit
line BL1 connected to a selected memory cell 10a shown in FIG. 15
with broken lines surrounding the selected memory cell 10a. Then
(or simultaneously therewith), a voltage is applied to a word line
WL1 being connected to a gate G of a cell selection transistor 11a
connected to the CB-RAM device 10a, so that the cell selection
transistor 11a is turned on. The bias voltage to be applied to the
selection bit line BL1 is set to have a positive value with respect
to a source line SL1 (is set to be negative in a case that an upper
electrode is the electrode B), and the absolute value thereof is
set to be equal to or slightly greater than the absolute value of a
voltage required for set.
[0092] The source line SL1 being connected to the selected memory
cell 10a can be brought to be at a reference potential, or a ground
potential 0V, for example, to create a current path from a bias
voltage of the bit line BL1 to a ground potential via the bit line
selection transistor 12, the cell selection transistor 11, and the
CB-RAM device 10a, and, in accordance with the ratio of a
resistance R of the CB-RAM device 10a in the high resistance state,
a channel resistance r of the cell selection transistor 11, and the
channel resistance of the bit line selection transistor 12, the
bias voltage is divided to the CB-RAM device 10 and the channel
resistance r' of the bit line selection transistor 12. The sum of r
and r' is smaller than R, and r and r' are set to be greater than
the resistance R' of the CB-RAM device 10 in the low resistance
state. In other words, they are set such that R'<r+r'<R is
satisfied. The resistance of the CB-RAM device 10a decreases from R
to R' at the instance of set, so that current flowing through the
CB-RAM device 10a immediately after the set is controlled by r+r'.
Thereafter, once the bias voltage is brought back to 0V, reset is
completed.
[0093] On the other hand, while reset being the switching process
from low resistance to high resistance is also carried out with the
same procedure as that for the set process, the point to be kept in
mind is that the bias voltage (with respect to the source line BL1)
to be applied to the selection bit line BL1 will have positive and
negative reversed with respect to the case of the set. In other
words, in a case of an upper electrode being the electrode A, the
bias voltage to be applied to the selection bit line BL1 is set to
have a negative value with respect to the source line SL1. For
example, the selection bit line BL1 is set to be at the ground
potential 0V and the source line SL1 is set to have a positive
value. Thereafter, once the bias voltage is brought back to 0V, the
reset is completed.
[0094] For reading, the gate voltage is adjusted such that both
channel resistances of the cell selection transistor 11a and the
bit line selection transistor 12 are brought to be sufficiently
smaller than the value r of the low resistance of the CB-RAM device
10a, and detecting current flowing when a prespecified voltage is
applied allows the resistance of the CB-RAM device 10a to be
determined.
[0095] [Variations of Embodiment]
[0096] The invention is not limited to the above-described
embodiments, so that a variety of variations are possible.
[0097] For example, such as material for an oxide film, film
forming conditions, and a solvent to be used being recited in the
above-described embodiments merely show one example, so that
modifications or changes are possible as needed in accordance with
the common general technical knowledge of a person skilled in the
art.
[0098] Moreover, while an example being applied as a CB-RAM, or, in
other words, a memory device, is shown in the above-described first
embodiment, the invention is to provide a general technique to
control diffusion of electrode A-constituting atoms in the
insulator layer, the above-mentioned application is not to be
limited to a memory, so that it can be applied to various
devices.
[0099] [Configuration of Switching Device]
[0100] For example, the above-described configuration of the memory
cell 10 can be used as a switching device. In other words, the
switching device comprises: a first metal layer including an
electrochemically active and easily ionizable metal; a second metal
layer including an electrochemically stable metal; an insulating
layer being sandwiched between the first metal layer and the second
metal layer and having a pore communicating from a first surface
being in contact with the first metal layer to a second surface
being in contact with the second metal layer; and an electrolyte
layer being impregnated in the pore of the insulator layer, wherein
the electrolyte layer contains a mixed ionic liquid in which is
mixed a solvate ionic liquid, and a low-viscous ionic liquid being
an ionic liquid having a viscosity coefficient smaller than that of
the solvate ionic liquid. In other words, the configuration thereof
is the same as that of the memory cell 10 shown in FIG. 2 as
described previously. Conduction/non-conduction between the first
metal layer 1 and the second metal layer 2 is controlled in
accordance with the polarity of a voltage applied between the first
metal layer and the second metal layer, functioning as a switching
device.
DESCRIPTION OF REFERENCE NUMERALS
[0101] 1 FIRST METAL LAYER [0102] 2 SECOND METAL LAYER [0103] 3
INSULATOR LAYER [0104] 3a PORE [0105] 3b FIRST SURFACE [0106] 3c
SECOND SURFACE [0107] 4 ELECTROLYTE LAYER [0108] 11 CELL SELECTION
TRANSISTOR [0109] 12 BIT LINE SELECTION TRANSISTOR [0110] WL, WL1,
WL2: WORD LINE [0111] BL, BL1, BL2: BIT LINE [0112] SL, SL1, SL2:
SOURCE LINE
* * * * *